Currently, no practical method exists for direct measurement of force production from individual muscles. Manual muscle tests do not give an accurate estimate of muscle strength. Measurements of joint torque are inadequate because several muscles often contribute to torque development. Implantation of a buckle transducer on a tendon is highly invasive and impractical for regular use. The integrated electromyogram is customarily used to provide quantification of muscle contraction. However, the problem remains that the electromyographic activity cannot provide a quantitative measure of muscle tension under dynamic conditions. An alternative, measurable parameter related to muscle force is intramuscular pressure. Commercially available intramuscular pressure transducers are too large for optimum comfort. Microsensor technology is now available to construct transducers that are approximately the same size as the fine wires used for electromyographic analysis. The overall objective of this project is to develop and test a fiber optic microsensor that can be used for routine, clinical measurement of muscle function. The specific aims of this study are a) to continue development of a fiber optic microsensor to measure intramuscular pressure, b) to determine the relationships between intramuscular pressure and muscle tension under dynamic conditions for normal muscle in an animal model, c) to develop a finite element model of intramuscular pressure in order to establish a theoretical basis for understanding the experimental measurements, and d) to perform in-vivo human experiments to evaluate the ability of intramuscular pressure to reflect the recruitment of motor units, number of active motor units, and the size of the compound muscle action potential. The hypothesis is that intramuscular pressure is directly related to two independent phenomena; namely, passive elongation and active contraction of muscle fibers. Successful development of this sensor will result in a powerful new tool. The ultimate goal is to use this microsensor for clinical decision making to improve the mobility of patients with neurogenic disorders (e.g. motor neuron disease, peripheral neuropathy), disorders of neuromuscular transmission (e.g. myasthenia gravis, Lambert-Eaton syndrome) and myopathies (e.g. muscular dystrophies, polymyositis, metabolic myopathies).